A. Field of the Invention
This invention relates to cutting tools used for working hard or refractory materials. More particularly, the invention relates to an improvement in drill bits of the type having sintered diamond or boron nitride cutting edges, and a method of making the improved bits.
B. Description of Background Art
Cutting tools used to work hard or refractory materials have a relatively short life. Also, tools such as drill bits used in drilling or forming composites containing boron or carbon fibers used in the aircraft industry, as well as in other industries requiring high-strength, light-weight materials, experience rapid wear. Even the drilling of aluminum alloy engine blocks rapidly wears out steel drill bits.
In an effort to increase the wear resistance and life of steel twist drill bits used in applications of the type described above, as well as in the fabrication of such materials as fiberglass impregnated epoxy printed circuit boards, the tips of the bits are typically fabricated from a very hard substance such as tungsten carbide. However, even tungsten carbide is abraded fairly rapidly by composites such as printed circuit boards containing glass fibers. Thus, the bits must be re-sharpened or re-pointed relatively frequently, and eventually wear out. Therefore, even though tungsten carbide tipped drill bits last longer than all-steel bits, the downtime associated with re-sharpening and replacing carbide tipped drill bits is still a problem.
In an effort to further increase the life of drill bits used to work refractory materials, bits having harder cutting surfaces consisting of diamond or cubic boron nitride have been developed. One class of bits of this type has veins of polycrystalline diamond (PCD) or boron nitride (PCBN) material formed in the bit. Bits of this type generally employ grooves cut into a steel or tungsten carbide blank. Fine grains of PCD are packed into the grooves, and then subjected to great heat and pressure. This process causes the diamond grains to fuse to one another and to the carbide walls of the groove, thereby forming veins of hardened material. Drill bits of the type discussed above are disclosed in the following U.S. Pat. Nos.:
Bunting, et al., 4,713,286, Dec. 15, 1987, Printed Circuit Board Drill and Method of Manufacture:
Discloses a drill bit for printed circuit boards made from a conically pointed carbide blank brazed to a shank. A rectangular or hour-glass end view shape slot is cut longitudinally inward into the point, filled with diamonds or cubic boron nitride grains, which are then fused to the blank by the application of heat and pressure. Helical flutes are then cut into the blank, adjacent the sintered region, which forms the web, cutting lips, margins and lands of the finished drill bit.
Bunting, 4,762,445, Aug. 9, 1988, Composite Sintered Twist Drill:
Discloses a drill bit in which a pair of non-aligned slots are cut into the tip of the drill, parallel to opposite sides of the web, filled with PCD material or boron nitride, and sintered under heat and pressure. In one embodiment the planes of the slots are parallel to the longitudinal axis of the drill bit. In a second embodiment the two slots are laterally displaced, and converge inwardly along the helix angles of the bit, and are joined at the tip by a longitudinally disposed central bore filled with sintered material.
Packer, 5,031,484, Jul. 16, 1991, Diamond Fluted End Mill:
Discloses an end mill made of tungsten carbide having grooves formed in the outer edges of helical cutting flutes formed therein. The grooves are packed with polycrystalline diamond powder or cubic boron nitride powder, which is sintered to the walls of the groove by the application of heat and pressure.
In another approach to providing a drill bit having cutting edges made of an extremely hard material such as diamond or cubic boron nitride, inserts containing PCD or PCBN are first fabricated, and then brazed into slots formed in a drill blank. Typically, these inserts take the form of a laminated, three layer structure or "sandwich" having two parallel outer walls made of a hard brazeable metal, such as tungsten carbide, and an inner layer of sintered diamonds or boron nitride grains. In plan view, inserts of this type usually have the shape of a chevron or pentahedron, the upper vertex of which forms the point of drill bit holding the insert. Such inserts are described in catalog #SD 1051 5M 64/91 published by Megadiamond, a Smith International, Inc. company, 275 West 2230 North, Provo, Utah 84604.
A disadvantage of the chevron or pentahedron sandwich form for PCD or PCBN inserts is that a relatively large portion of the PCD layer is remote from the cutting edge. Since the PCD material is quite expensive, it would be desirable to have a drill bit insert in which PCD material is located primarily on a cutting edge of a drill bit, rather than in an unused location. In an apparent effort to provide PCD drill bit inserts in which a minimum of PCD material is required, a modified PCD insert has been made available by De Beers Industrial Diamond Division, Charters Sunninghill Ascot, Berkshire, SL5 9PX England.
In De Beers'Syndite product announcement, titled Syndite, New Inserts For PCD-Tipped Drill Bits, a modified PCD insert for twist drills is disclosed, in which PCD material is confined largely to one oblique upper edge wall or "rooftop" of a pentahedral-shaped insert. Inserts of this type are fabricated by forming a surface layer of PCD material on a round or square base of brazeable matrix metal, and then cutting a rectangular or pentahedral-shaped piece from the PCD blank. In the case of the pentahedral cut, the base of the pentahedron is angled to the parallel upper PCD layer of the blank, so that PCD material lies on just one oblique upper wall or rooftop of the pentahedron. Two such pentahedrons are then placed side by side in a longitudinally disposed rectangular slot formed in the end of a drill bit blank. The PCD walls are on opposite sides of the bit, which is then machined to form a drill bit having two diametrically opposed, PCD cutting lips.
All of the drill bits employing PCD or PCBN cutting edges disclosed above have one or more less than optimum characteristics.
For example, bits having integral veins of fused PCD material are difficult and costly to fabricate, in part because of their complicated geometry.
Bits using chevron or pentahedral sandwich inserts require the grinding away of a substantial portion of one tungsten carbide outer face of a sandwich, in the formation of cutting lips. Moreover, the geometry of these inserts results in the depletion of PCD material after a relatively small number of re-grinds or repointings of a bit employing the insert.
In bits using a pentahedral insert having a PCD layer on an oblique surface or rooftop of the pentahedron, the exposed PCD layer is supported by fused PCD material only on its lower surface, rather than on a base and one side, as is the pentahedral or chevron sandwich type insert. Thus, drill bits using a pentahedral insert having a single PCD surface layer tend to be less rugged than bits using PCD sandwich inserts.
In view of the limitations of prior art drill bits employing PCD or PCBN veins or inserts described above, improved drill bits employing sintered PCD inserts, and methods of making the bits were conceived by the present inventor.